本文将深入解析Intel iret instruction。

1. Prerequisite

建议读者阅读下浅谈栈 与浅谈tss。

2. Materials

最权威的资料，值得读者细品。

  • Intel SDM Vol1 CHAPTER 6 PROCEDURE CALLS, INTERRUPTS, AND EXCEPTIONS
  • Intel SDM Vol2 INSTRUCTION SET REFERENCE

3. Theory

4. Example in kvm unit test

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asm volatile (
		/* Prepare kernel SP for exception handlers */
		"mov %%rsp, %[rsp0]\n\t"
		/* Load user_ds to DS and ES */
		"mov %[user_ds], %%ax\n\t"
		"mov %%ax, %%ds\n\t"
		"mov %%ax, %%es\n\t"
		/* IRET into user mode */
		"pushq %[user_ds]\n\t"
		"pushq %[user_stack_top]\n\t"
		"pushfq\n\t"
		"pushq %[user_cs]\n\t"
		"pushq $user_mode\n\t"
		"iretq\n"

		"user_mode:\n\t"
		/* Back up registers before invoking func */
		"push %%rbx\n\t"
		"push %%rcx\n\t"
		"push %%rdx\n\t"
		"push %%r8\n\t"
		"push %%r9\n\t"
		"push %%r10\n\t"
		"push %%r11\n\t"
		"push %%rdi\n\t"
		"push %%rsi\n\t"
		/* Call user mode function */
		"mov %[arg1], %%rdi\n\t"
		"mov %[arg2], %%rsi\n\t"
		"mov %[arg3], %%rdx\n\t"
		"mov %[arg4], %%rcx\n\t"
		"call *%[func]\n\t"
		/* Restore registers */
		"pop %%rsi\n\t"
		"pop %%rdi\n\t"
		"pop %%r11\n\t"
		"pop %%r10\n\t"
		"pop %%r9\n\t"
		"pop %%r8\n\t"
		"pop %%rdx\n\t"
		"pop %%rcx\n\t"
		"pop %%rbx\n\t"
		/* Return to kernel via system call */
		"int %[kernel_entry_vector]\n\t"
		/* Kernel Mode */
		"ret_to_kernel:\n\t"
		"mov %[rsp0], %%rsp\n\t"
		:
		"+a"(rax),
		[rsp0]"=m"(tss[0].rsp0)
		:
		[arg1]"m"(arg1),
		[arg2]"m"(arg2),
		[arg3]"m"(arg3),
		[arg4]"m"(arg4),
		[func]"m"(func),
		[user_ds]"i"(USER_DS),
		[user_cs]"i"(USER_CS),
		[user_stack_top]"r"(user_stack +
				sizeof(user_stack)),
		[kernel_entry_vector]"i"(RET_TO_KERNEL_IRQ)
		:
		"rsi", "rdi", "rcx", "rdx");

为了能进入user mode，在内核态，进行了如下操作：

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/* Load user_ds to DS and ES */
"mov %[user_ds], %%ax\n\t"
"mov %%ax, %%ds\n\t"
"mov %%ax, %%es\n\t"
/* IRET into user mode */
"pushq %[user_ds]\n\t"
"pushq %[user_stack_top]\n\t"
"pushfq\n\t"
"pushq %[user_cs]\n\t"
"pushq $user_mode\n\t"
"iretq\n"

5. 总结

掌握住iret instruction的Operation即可！
详情当然是参考Intel SDM Vol2 INSTRUCTION SET REFERENCE!

建议详细阅读下Switching to User-mode using iret！

参考资料:

  1. Switching to User-mode using iret
  2. CPU Switches from Kernel mode to User Mode on X86 : When and How?
  3. jamesmolloy.co.uk User mode